Among the processes for preparing a microporous film from polyolefin, a wet process comprises mixing polyolefin with a diluent at a high temperature to form a single phase, which is then separated into polyolefin and the diluent during subsequent cooling step. Then the diluent is extracted to provide voids in the polyolefin film. According to the wet process, a microporous film with thin and even thickness can be produced and the physical properties of those film are also good, so that the film can be used as a battery separator of secondary batteries such as lithium ion batteries.
The wet processes for preparing a porous film are classified into solid-liquid phase separation and liquid-liquid phase separation, depending upon the way how the diluent is mixed with the polymer (resin), phase separated and forms pores. Both processes involves the same procedure until making a single phase by mixing the polymer and the diluent at a high temperature, but the difference of phase separation mechanism gives different properties of the microporous film finally obtained.
In case of solid-liquid phase separation, there occurs no phase separation until the polymer is crystallized via cooling to be solidified. In other word, as the polymer chains are crystallized, they force the diluent outside the crystals to cause phase separation. Thus, the size of the phase formed through the phase separation would be very small like the size of the polymer crystals, and the structure (shape and size) of the phase separated cannot be diversely controlled. The separator formed via this process cannot be applied to recently developed secondary battery which has high capacity due to low permeability. To improve the mechanical strength, there is no other way but to essentially raise the molecular weight of the polymer resin by mixing ultrahigh molecular weight polyolefin or the like. However, ultrahigh molecular weight polyolefin is expensive and difficult to be mixed, with largely increasing the processing load. As the representative composition of solid-liquid phase separation, polyolefins mixed with paraffin oil or mineral oil are widely known, as were disclosed by U.S. Pat. Nos. 4,539,256; 4,726,989; 5,051,183; 5,830,554; 6,245,272; 6,566,012, etc.
In case of liquid-liquid phase separation, before the polymer is crystallized and solidified, phase separation occurs between the polymer and the diluents in liquid state due to thermodynamic instability at a temperature higher than the crystallization temperature of the polymer. Thus, the shape and size of the phase formed through liquid-liquid phase separation would be changed according to the conditions of phase separation. Thus, the liquid-liquid phase separation is advantageous in that temperature of liquid-liquid phase separation and size of the phase can be controlled depending on the combination of polymers and diluents, and that the size of the phase can be diversely controlled by the difference between the thermodynamic liquid-liquid phase separation temperature and the temperature at which actual phase separation occurs, and by the retention time of individual steps. In case of a microporous film produced via a liquid-liquid phase separation process, the pore size can be controlled unlike solid-liquid phase separation and can be at least several times as large as that of a microporous film produced via solid-liquid phase separation.
A liquid-liquid phase separation process is not widely used because of difficulties in processing, in spite of the advantages described above. The first difficulty of a liquid-liquid phase separation process is that the polymer and diluent should be mixed to be single phase first at a temperature higher than the liquid-liquid phase separation temperature in the extruder, and then phase separation should be taken place. In other word, in the first stage to make a single phase, the temperature should be above the liquid-liquid phase separation temperature. During raising the temperature up to liquid-liquid phase separation temperature, thermodynamic mixing doesn't occur. Therefore, when a general extrusion process is applied, the extrusion temperature might be very high with extruding condition being complicated, to result in reducing productivity. Moreover, excessive oxidation of composition occurs during the extrusion, to cause deterioration of physical properties and durability of the microporous film produced. The second difficulty of the process is to find a diluent having suitable liquid-liquid phase separation condition (temperature) for the processing, which is not simple.
U.S. Pat. No. 4,247,498 discloses a wide variety of combinations of polymers and diluents, which can undergo liquid-liquid phase separation. It is described that diluent is extracted from the composition which was separated by liquid-liquid phase separation, to manufacture film with various thicknesses. U.S. Pat. No. 4,867,881 describes a process for manufacturing an oriented microporous film by stretching, extracting, drying and heat setting the composition which was separated by liquid-liquid phase separation. However, neither of the patents could suggest a method to maximize the effect of liquid-liquid phase separation through effective extrusion of the composition.